The fixing device of the image-forming apparatus attaches toner to an electrostatic latent image formed by an optical device and transfers a toner image to a sheet of copy paper and fixes the toner image thereto. In the fixing process, the toner image is passed between the fixing roller incorporating a heater and the pressure roller. Thereby a transferred image consisting of the toner image is fixed to the copy paper by means of thermal fusion.
The fixing roller is made of a soft metal and incorporates a linear or rod-like heater at its shaft center and formed in the shape of a cylinder. The shaft is projected from both ends thereof in a small diameter. The fixing roller is composed of a metal material such as aluminum or an aluminum alloy (A5056, A6063) excellent in its thermal conductivity. The surface of the fixing roller has lathe turning finish or abrasive finish. The surface of the fixing roller is coated with resin such as fluororesin or the like having a high nonadhesion. The surface the fixing roller is heated up to 180 to 250° C. by a heater.
The pressure roller is composed of an iron material or a soft material coated with silicone rubber or the like and rotatingly operated with the pressure roller pressing the copy paper against the fixing roller. The pressure roller is heated up to 70 to 150° C. by heat transferred from a heating roller. Alternatively similarly to the fixing roller, the pressure roller is heated up to 150 to 250° C. by a heater incorporated inside it.
Rollers such as the fixing roller and the pressure roller to be heated by heat transferred from the heater incorporated therein or from other members are “heat roller”.
The heat roller to be heated up to high temperatures is rotatably supported by a housing via a rolling bearing consisting of a deep groove ball bearing at both ends of its shaft. A heat insulation sleeve made of synthetic resin or the like is interposed between the rolling bearing and the shaft of the heat roller. This is to prevent heat from escaping from portions of the rolling bearing disposed at both ends of the shaft of the heat roller when the heat roller is heated so that the temperature distribution along the axial direction of the heat roller does not become nonuniform and prevent the bearing from deteriorating owing to high temperatures.
An art of using a sliding bearing made of resin to support the heat roller is known. As an example of the art, the sliding bearing is formed of synthetic resin such as polyphenylene sulfide (PPS) resin, polyamide resin, polyamide-imide resin, polyimide (PT) resin, and polyether ether ketone resin. More specifically, the art of bonding fluororesin to the sliding surface of the ring-shaped bearing body made of the PPS resin excellent in its heat resistance and mechanical strength and the art of integrally molding the PPS resin mixed with fluororesin into the sliding bearing are known (see patent document 1).
When the sliding bearing made of the resin is used, generally, the heat insulation sleeve is not interposed between the sliding bearing made of the resin and the shaft of the heat roller because the sliding bearing made of the resin has heat-insulative. Normally the rolling bearing is used for the heat rollers of the fixing devices of middle-class and high-class image-forming apparatuses, whereas the sliding bearing made of the resin is used for the heat roller of the fixing device of an image-forming apparatus in widespread use.